1. Field of the Invention
The present invention relates to a driving method of an AM-OLED (Active Matrix Organic Light Emitting Diode) panel, and more particularly, to a pixel driving circuit of an electro-luminescent display device that is capable of preventing changes in brightness due to temperature changes in the panel or a performance deterioration of the OLED and a driving method thereof.
2. Discussion of the Related Art
Generally, pixel structures for AM-OLED displays may be broadly classified into voltage-driven pixels, current-driven pixels and digitally-driven pixels according to their driving method.
The voltage-driven pixel has a structure such that a voltage may be driven at a high speed that is similar to a TFT-LCD (Thin Film Transistor-Liquid Crystal Display) driver LSI (Large Scale Integrated Circuit). Accordingly, it is easy to implement the driver LSI. However, the voltage-driven technique has drawbacks in that the brightness between the upper and lower portions of a display panel may be different due to a drop in a pixel power supply voltage, and also crosstalk noise may be generated. Even though non-uniformity of a threshold voltage is in the driving transistor may be compensated to a certain degree, it is difficult to compensate for variations in election mobility in the driving transistors.
The current-driven pixel has a structure such that a current driven. Accordingly, it is easy to compensate for changes in the characteristics of the TFT and is also possible to compensate for a IR drop in the power supply voltage. But, it is difficult to drive a current within a short row line time because a data line presents a large parasitic load upon application of a current having a low gradation.
The digitally-driven pixel structure is very sensitive to the deterioration or characteristic changes in the OLED material.
FIG. 1 is a schematic circuit diagram of a pixel circuit for driving a current in an electro luminescent display device in accordance with the related art.
As shown in FIG. 1, the pixel circuit for driving a current in the electro-luminescent display device includes: a PMOS-FET (P-channel Metal Oxide Semiconductor Field Effect Transistor) driving transistor (T1) and a PMOS FET switching transistor (T2) that are connected in series between a power supply terminal (VDD) and an organic light emitting diode (OLED) in order to supply a drive current to the OLED; a storage capacitor (Cstg) connected between a source terminal and a gate terminal of the PMOS FET driving transistor (T1); a PMOS FET switching transistor (T3) the source terminal of which is connected to a data line (DL) and a drain terminal of which is connected to the gate terminals of PMOS FET transistors (T1), (T2), and having its gate connected to a first gate signal terminal (GATE1); and a PMOS FET switching transistor (T4) the source terminal of which is connected to the data line (DL) and the drain terminal of which is connected to the drain and source terminals of PMOS FET transistors (T1), (T2), respectively, and having its gate connected to a second gate signal terminal (GATE2).
The operation of the pixel circuit of FIG. 1 will be described as follows. First, when the gate signal terminals (GATE1), (GATE2) are at a ‘low’ level, the switching transistors (T3), (T4) are turned on, respectively. Accordingly, driving transistor (T1) sinks a current from the power supply terminal (supplied from a data driving unit). The current flowing at this time is uniformly sunk, and accordingly the same amount of the current flows to all pixels.
Thus, a voltage corresponding to the sunk current is charged on the storage capacitor (Cstg). But, the characteristics of the driving transistors (T1) in each pixel are different. So, voltages are charged on the storage capacitors (Cstg) in each pixel having different levels.
Thereafter, when gate signal terminals (GATE1), (GATE2) assume a ‘high’ level, the switching transistors (T3), (T4) are turned off, respectively. At this time, driving transistor (T1) supplies the current corresponding to the voltage charged in the storage capacitor (Cstg) towards the organic light emitting diode (OLED), thereby obtaining an excellent uniformity.
As mentioned above, the OLED is arranged in a matrix to form a panel, and an OLED panel having the above structure is used for displaying an image. The OLED panel is driven by a power supply voltage (VDD) supplied from a DC/DC converter (not shown).
Also, even though PMOS FET transistors are used as the transistors (T1-T4) in the above description, NMOS FET transistors can be also used as well.
The pixel driving circuit of the electro-luminescent display device in accordance with the related art has a problem in that the current supplied to the OLED panel is not properly controlled. Accordingly, the temperature increases and the power supply voltage drops below a predetermined level that causes the OLED panel not to operate properly that results in a deteriorated image quality.